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Thread: hardware failure

  1. #1
    Junior Member
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    hardware failure

    Hi everyone,
    I am using since few years a lot of teensy, and I would like to understand what I can do for the failure I have met now: for teensy 3.6

    - When I plug the microUSB power supply, the voltage drops to 4 Volt approximatly and the current reach 0.5A or even 1A. (I do not understand why it could reach so much since there is a 500mA fuse...)

    I guess there is either something connected to ground ( I doubt it because I did not touch anything). Maybe the only bad thing I did is that I plug a 5V power supply to Vin at the same time with the microUSB power supply while not cutting the Vusb and Vin pad.
    Maybe a component is dead or is dying, My guess if for the LDO.
    I can feel overheat at the fuse and the double diode 1A.

    Do you have some idea to understand where is the component I could change?

    Cheers

  2. #2
    Member
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    is any chip, resistor, etc on the pcb getting hot to the touch?

  3. #3
    Junior Member
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    Hi,

    yes, the fuze 500mA and the double diode (the two components just after the uUSB)

    thanks

  4. #4
    What is the resistance between VUSB and GND? 3.3V and GND?

    Does the 3.6 work and not draw too much current when powered from the 3.3V input?

  5. #5
    Senior Member PaulStoffregen's Avatar
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    From only the info here, I can't possibly guess what went wrong. But I can answer this...

    Quote Originally Posted by phonoptics View Post
    and the current reach 0.5A or even 1A. (I do not understand why it could reach so much since there is a 500mA fuse...)
    The "fuse" is actually a PTC resistor. When it gets hot, the resistance increases until (hopefully) the increased resistance in series acts to limit the current flow. But this isn't an exact science. First of all, the actual temperature can depend on the room temperature and nearby air flow, so exactly how much current is needed to reach that stable point can vary.

    The PTC resistors aren't a high-precision device like we're used with ordinary resistors. Physically they're made from non-conducting polymers or ceramics that have a high thermal expansion coefficient. Metal particles are embedded into the material as it's formed, at just enough density so that conductive paths are formed by the particles which happen to touch each other within the material. As it gets hot and thermally expands, fewer of the particles touch, which rapidly increases the resistance.

    Regular fuses, where a metal filament burns, also suffer from the same types of inaccuracies. It's rated to pass 0.5 amps of current. But 0.51 amps is so similar that it probably won't make any difference. It takes a lot more current than the rated value to blow the fuse. Many fuses take about double the rated current to blow, and they tend to have specs about how quickly they will blow as you increase the current. It can take a surprising amount of time to blow at only 2X the rated current.

    Very precise over-current circuits are possible using shunt resistors, opamps, precise voltage reference, and a big mosfet transistor to turn off the current. But that sort of circuitry adds quite a lot of cost and takes up space.

    With PTC resistors and real fuses, you just don't get highly precise current thresholds.

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